1. Field of the Invention
The present invention relates to an optical waveguide for propagation of far-infrared radiation having a frequency in the vicinity of 1 THz and a wavelength in the vicinity of 300 μm, which is so-called terahertz waves.
2. Discussion of Background
Far-infrared radiation having a frequency in the vicinity of 1 THz (wavelength: 300 μm) is called terahertz waves and has attracted attention very much in recent years. Namely, by means of terahertz waves, phonons or excitons of a substance can be directly excited, whereby observation of various physical phenomena can be carried out and is expected to be prospective as a new measuring method. A terahertz spectroscopy of many biological materials or polymer materials has already been carried out.
However, in the current terahertz spectroscopy, a catoptric system employing e.g. a parabolic mirror in free space, is constituted to have a sample irradiated, in many cases. Therefore, the influence of the absorption of terahertz waves by the atmosphere is substantial, and in order to avoid such influence, the entire apparatus is put in a chamber, which is then evacuated or filled with e.g. a nitrogen gas, thus requiring a large scale set-up. Therefore, various studies have been made with respect to optical waveguides capable of propagation of terahertz waves with a low loss, or with respect to materials to be employed for such optical waveguides. Recently, those employing plastic ribbons as waveguides (Non-Patent Document 1) or those employing metal wires as optical waveguides (Non-Patent Document 2) have been reported.
Such optical waveguides are required to have a high coupling efficiency and a broad bandwidth to terahertz waves, and as one method to satisfy such requirements, a photonic crystal fiber (PCF) has been studied (Non-Patent Documents 3 and 4).
Photonic crystal has such a structure that the refractive index changes in a period shorter than the wavelength of light to be used, and light propagating therethrough is controlled by a quantum optic effect. For example, by a terahertz integrated optical system having a Teflon® PCF waveguide and a lens duct combined, as reported by Sarukura et al., it is possible to suppress a coupling loss of terahertz waves generated in the lens duct as a terahertz emitter to an extremely small level, at the time of introducing them into the Teflon PCF waveguide (Non-Patent Document 5).
However, the transmission loss of the Teflon PCF waveguide itself is relatively large, and its application is rather limited.
Non-Patent Document 1: R. Mendis and D. Grischkowsky, J. Appl. Phys. 88, 4449 (2000).
Non-Patent Document 2: K. Wang and D. Mittleman, J. Opt. Soc. Am. B 22, 2001 (2005).
Non-Patent Document 3: H. Han, H. Park, M. Cho and J. Kim, Appl. Phys. Let. 80, 2634 (2002).
Non-Patent Document 4: M. Goto, A. Quema, H. Takahashi, S. Ono, and N. Sarukura, Jpn. J. Appl. Phys. 43, L317 (2004).
Non-Patent Document 5: G. Diwa, A. Quema, E. Estacio, R. Pobre, H. Murakami, S. Ono, and N. Sarukura, Appl. Phys. Let. 87, 15114 (2005).
Presently known materials transparent to terahertz waves are very limited, and it has been difficult to obtain the required performance by selecting the material. Further, it is also difficult to search for a novel material.